1. Field
This patent specification relates generally to image forming apparatuses, and more specifically to the apparatus provided with a plurality of visible image forming units each including development means for developing to visualize latent images held on latent image bearing members, and a light scanning means for forming the latent images through light beam scanning to be held onto the latent image bearing members of the visible image forming units.
2. Discussion of the Background
A number of apparatuses are known capable of forming images by creating images to be rendered visible on a latent image bearing member such as photoreceptor and by transferring onto an intermediate transfer member to be superimposed as a resultant images. Such apparatus may be exemplified by including only one visible image forming means comprising the latent image bearing member and plural developing units associated therewith.
In order to form proper superposed images by such an image forming apparatus, in general, the surface of intermediate transfer member has to be rotated repeatedly as many times as the number of image superposition.
As an example, in order to form a full-color image by superposing four visible images in the colors of yellow, magenta, cyan, and black (hereinafter referred to as Y, M, C, and K, respectively), Y image is first obtained by developing a latent image held on a developing means in use for Y color, and subsequently transferred onto the intermediate transfer member during a first round of its rotation.
Thereafter, M, C, and K images created on respective latent image bearing members are transferred sequentially onto intermediate transfer member during second, third, and fourth rounds of the rotation respectively, and a full-color image is finally obtained.
The latent image bearing member serves to hold a latent image generated by light beam scanning. The light beam scanning, in turn, is divided broadly into two, one in the direction of surface displacement of the latent image bearing member (i.e., vertical scanning direction) and the other the direction perpendicular thereto (main scanning direction).
The scanning in the main scanning direction is carried out with light beams repeatedly deflected to be scanned with an extremely short period along that direction by deflecting means including a rotating polygonal mirror and so on.
It is noted whether or no a light beam is located at a write initiation point is determined based on a beam detection signal which is output by a beam detection means provided to detect the beam at a predetermined location in the main scanning direction.
The timing for initiating light scanning, however, has to be determined after considering the location not only in the main scanning direction but also in the vertical scanning direction.
Therefore, the write initiation point in the vertical scanning direction is suitably determined by utilizing a mark detection signal output by a mark beam detection means for detecting a base mark which is affixed at a predetermined location in the direction of surface displacement of the intermediate transfer member.
To be more specific, the timing for initiating light scanning with respect to both the main and vertical scanning directions is adjusted by initiating the scanning based on the beam detection signal detected immediately after the generation of the mark beam detection signal.
In the light beam scanning carried out repeatedly along each rotation of the intermediate transfer member, however, shift in proper timing (or phase) of the scanning in the main scanning direction cannot sufficiently be avoided in general.
For example, assumed herein are a circumferential length of 600 mm and a linear speed of 40 mm/sec for the intermediate transfer member, and a scanning speed of 1000 time/sec in the main scanning direction for the light scanning means (which gives rise to 15000 for the number of beam scanning during one round of the transfer member rotation).
Also assumed that the light beam happens to be located at a write initiation point at the moment when a mark is detected. In this case, light beam scanning for the first round of rotation can be initiated at the noted moment of the mark detection.
In addition, the scanning continues in principle such that the light beam is located again at the write initiation point after the first round of rotation and that light beam scanning during the second round of the rotation (i.e., the fifteen thousand and first round of the main direction beam scanning) is then initiated.
In practice, however, this is not the case since the location of light beam in the main scanning direction is shifted away from the write initiation point when the first round of transfer member rotation is completed, because of an error in the circumferential length, and change in linear speed of intermediate transfer member. As a result, the difficulties arise as evidenced by the shift in phase of the scanning in the main scanning direction, as described earlier.
This shift brings about a change in the period between the mark detection and the initiation of light scanning based on the succeeding beam detection, thereby resulting in shear in superposed visible images (color shift in multiple-color images) in the vertical scanning direction.
In addition, a minute slippage may arise in a belt on driving roller in the case where a belt is used as an intermediate transfer means. In this case, the shift may arise with more ease in phase of the scanning in the main scanning direction each round of rotation compared with an intermediate transfer drum with less possibility of slippage.
Furthermore, although the case has been described above in which the scanning is carried out such that the light beam is located again at the write initiation point after the first round of rotation (or the number of repeated beam scanning is integer), other cases may also arise with a non-integer.
For example, if the abovementioned linear speed of the intermediate transfer member is altered to 42 mm/sec from the previous value of 40 mm/sec, the number of repeated beam scanning during one round of rotation is obtained as 14285.7. In this case, therefore, the shift in phase of light beam scanning in the main scanning direction is bought about regardless of the noted error in the circumferential length or a change in linear speed of intermediate transfer member.
As an image forming apparatus capable of alleviating the shear in superposed visible images caused by the noted shift in phase during the main direction scanning, an apparatus has been disclosed in Japanese Laid-Open Patent Application No. 10-202944.
This apparatus is adapted to control a polygon motor as the driving power source for a rotating polygonal mirror so as to adjust the phase of the rotation of polygonal mirror as a part of deflecting means to coincide with the signal output itop based on the difference in time between the signal itop and a signal BD. The signals itop and BD designate herein the mark detection, and the beam detection signals, respectively, as noted earlier.
By initiating light scanning for each round of the rotation of intermediate transfer drum with the apparatus, the shift in the phase of beam scanning can be resolved to a certain extent and the shear in superposed visible images due to the noted shift in phase during the main direction scanning can be suppressed effectively.
In order to form superposed images by such apparatuses, however, the surface of intermediate transfer member has to be rotated as many times as the number of image superposition, that is three times of the rotation for a triple-color image, for example. This comes to result a prolonged period of time of image formation coppered with the case of mono-color images which is carried out without image superposition.
On the other hand, another type of image forming apparatus for obtaining superposed images is also known previously, which is provided with as many visible image forming means as the number of the superposition each including a latent image bearing member and a developing means accompanied thereto.
For forming full-color images, for example, the noted apparatus is provided with four visible image forming means each in use for forming images of four colors Y, M, C, and K, respectively.
In this structure of the image forming apparatus, the superposition repeated as many times as required can be carried out during one or less round of the rotation of intermediate transfer means instead of rotating as many times as described earlier, and the aforementioned difficulty such as prolonged image forming period can be obviated, which is otherwise caused from rotating as many times as the number of image superposition.
This apparatus, however, may suffer from the shear in superposed visible images in the vertical scanning direction, when any phase shift is once caused among the plural visible image forming means in the main scanning direction.
As an image forming apparatus capable of alleviating such shear in superposed visible images in the vertical scanning direction, an apparatus has been disclosed in Japanese Laid-Open Patent Application No. 2000-94747.
This apparatus is adapted to perform light beam scanning using polygonal mirrors for exclusive use onto respective latent image bearing members of visible image forming means. In addition, each of the polygonal mirrors is provided with plural beam detection means corresponding thereto. Based on beam detection signals from the beam detection means, phase of the rotation of polygonal mirrors is controlled so as to synchronize the phase for respective mirrors.
By bringing through such synchronization the phase shift to coincide with each other among the plural visible image forming means in the main scanning direction, the shear of superposed images can be alleviated in the vertical scanning direction.
In such image forming apparatuses, however, a drawback arises in which the structure of light beam scanning means becomes unduly complicated by incorporating rotating polygonal mirrors, polygon motor, and motor control circuit and so on, for exclusive use for each visible image forming means.